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Fibocom FM-150 (SDX55) requires send-delay=0 in the TTY

For some reason, the AT-capable ttyUSB ports exposed by the module require send-delay=0 or otherwise they won't respond to the AT probing:

ModemManager[547813]: <debug> [1617146244.405905] [ttyUSB2/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146244.408585] [ttyUSB1/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146247.650906] [ttyUSB2/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146247.651291] [ttyUSB1/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146250.635123] [ttyUSB2/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146250.635703] [ttyUSB1/at] --> 'AT<CR>'
ModemManager[547813]: <debug> [1617146253.634308] [ttyUSB2/probe] port is not AT-capable
ModemManager[547813]: <debug> [1617146253.647306] [ttyUSB1/probe] port is not AT-capable
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